## Questions for a Cosmologist

A coworker and I were having a discussion about general relativity, and wandered down a tangent in which our understandings of the universe differed. We both agree that each others’ explanations are plausible without any hard data against which to test predictions (and we didn’t have real data to back up any of our claims). Does anyone want to weigh in on this debate? Have cosmologists considered this idea and come to a conclusion one way or the other?

My coworker claimed that space is expanding all the time, and that if you have two particles at rest relative to each other, the distance between them will nonetheless increase if you wait long enough (even while the particles remain at rest relative to each other). As a consequence, he claimed that there were regions of the universe so far away from us that the space in between was expanding faster than light could traverse it (that is to say, some regions are so far away that the added distance gained from the expanding space over the course of a year was more than a lightyear, so light from those regions would move toward us but never get any closer). Consequently, we would not be able to observe anything from these unreachable regions.

I posited that space was static (unless it is warped by gravity, but everything we discussed could have happened with massless particles, so I’m going to ignore gravity for the moment): if two particles are at rest relative to each other, I would expect the distance between them to remain constant for all time. I thought that any part of the universe was reachable if you waited long enough (not counting black holes, of course), and light from every part will eventually reach every other part (again, not counting black holes [1]). I noted that we can’t actually measure space: we can only measure the relative positions of particles which occupy space, and if the distance between them is increasing, then they’re not at rest relative to each other.

We devised an experiment to determine which view was correct. Edit: rubixsqube pointed out that “at rest with respect to each other” is ill-defined on these scales, so I have changed the experiment so that the particles no longer need to be at rest. Suppose we have two massless particles [2] that are not accelerating, and we send light signals from one to the other at regular intervals. In my universe, the receiving particle observes the signal to have a constant period and constant frequency. In my coworker’s universe, it observes that the period of the signal is lengthening (when plotted over time, it would trace out a parabola) and the light’s frequency is increasingly red-shifted as time goes on. Unfortunately, this experiment cannot actually be carried out because it would take an extraordinarily long time to get any conclusive data.

However, my coworker suggested that the cosmic microwave background radiation was evidence for his theory: he claimed that it started off as gamma rays and has been red-shifted for the entire lifetime of the universe by this expanding space. However, neither of us had any evidence or reason to believe that it began as gamma radiation (side note: why is it all one frequency? If it’s left over from the big bang, I would expect it to have all kinds of crazy frequencies, rather than the single, narrow band which we have today).

We also discussed Edwin Hubble’s observation that galaxies farther away from us are red-shifted more than the closer ones, but it was consistent with both our views (perhaps the space in between us and them is expanding; perhaps distant galaxies are really just moving away from us faster than nearby ones), and we were reduced to our thought experiment about periodic signals traveling between particles again.

It’s worth noting that if space is expanding and there are regions from which light cannot reach us, we would not be able to observe these areas. Indeed, there ought to be an edge to the visible universe, and we should be able to watch (given enough time, of course) objects crossing that edge and suddenly becoming unobservable. However, neither of us had ever heard of that happening.

I tried reading up on some of this stuff, but couldn’t find any discussions of this particular topic. Is there a knowledgeable person who can comment on these things? Even if you’re not knowledgeable, you’re welcome to throw in your opinion.

[1] I’m aware of Hawking radiation, but I’m going to ignore it, too. None of this applies to black holes, so stop trying to find corner cases where it might apply.
[2] or massive particles; it really shouldn’t matter as long as you take the gravity into account and you don’t have event horizons or any crazy stuff like that. We used massless particles because it was easier.

1. It’s like, your mind is expanding, man.

Sorry, that’s all I got.

2. inferno0069 says:

This looks relevant: http://en.wikipedia.org/wiki/Metric_expansion_of_space
(I got to it from http://en.wikipedia.org/wiki/Hubble%27s_law and to that from http://en.wikipedia.org/wiki/Big_Bang )

It’s worth noting that if space is expanding and there are regions from which light cannot reach us, we would not be able to observe these areas. Indeed, there ought to be an edge to the visible universe, and we should be able to watch (given enough time, of course) objects crossing that edge and suddenly becoming unobservable. However, neither of us had ever heard of that happening.

As I interpret this, they wouldn’t suddenly become unobservable but rather they’d be slowly redshifting towards zero-energy light emissions, from our point of view. However, we’d continue being able to observe this light forever; it wouldn’t completely disappear until infinite time had passed.

3. janna says:

I feel like my understanding had been more in line with yours than your coworker’s, but I’m not sure. My dad is a physics and astronomy prof, so I can pass this on to him, as I believe he keeps up on the current theories pretty well.

4. rubixsqube says:

But I need to grab a book and do some thinking so that I can answer it with some intelligence. The very, very short answer is that all of this is quite complicated, but you’ve touched upon a bunch of really good points. The big problem with cosmology is that you really can’t say that any two points are “at rest” with respect to each other. Within a galaxy, we can say that things are at rest with respect to each other because they are gravitationally bound, and this gravity works against the hubble expansion. But if you have two points that are weakly bound by gravity, they’ll be moving away from each other as described by the Hubble Law.

Also, and again, I will answer in more detail later providing that I can grab my excellent cosmology book that I loaned to Michael (seriously, if you’re interested in it, buy Barbara Ryden’s “Introduction to Cosmology”), but there are places that are expanding away from us higher than the speed of light, which indicates that yes, these objects, from our rest point, are moving away at a speed greater than c. This does not violate Einstein’s theory of relativity, because that’s only defined to be in a rest frame, but when you get to expanding space, you throw a lot of that out the window. This, then, defines an edge to the visible universe, true. Ok, I should get back to work, but here are some complicated “answers” to some questions, on the page written by my cosmology prof, who’s one of the world’s foremost experts on the subject:

http://www.astro.ucla.edu/~wright/cosmology_faq.html

• Alan says: